1. Field of the Invention
This invention is in the field of protecting lead anodes from corrosion during metal-electroplating processes. More particularly, this invention provides a method and composition for electroplating chromium, using lead or lead-containing anodes under conditions which produce adherent, bright chromium deposits at high efficiencies, where cathodic low-current-density etching is substantially reduced in comparison with existing high-efficiency catalyst systems. The invention further provides a composition for the replenishment of exhausted or depleted plating baths while diminishing anode corrosion.
2. Description of the Prior Art
Several advantages of certain short-chain alkylsulfonic acids in chromium electroplating have been described for both decorative and functional systems. U.S. Pat. No. 3,745,097 to Chessin, assigned to the same assignee as this invention, discloses decorative electroplating baths containing alkylsulfonic or haloalkylsulfonic acids in combination with certain carboxylic acids to produce bright, iridescent chromium surfaces on the articles plated. In U.S. Pat. No. 4,588,481, Chessin et al. further disclose functional chromium electroplating processes which use baths containing alkylsulfonic acids having a ratio of sulfur to carbon of 1/3 or greater, but free of carboxylic acids; the processes result in hard, adherent chromium deposits produced at elevated temperatures and high efficiencies without cathodic low-current-density etching. However, the chromium-plating baths taught by U.S. Pat. No. 4,588,481, while yielding the high-efficiency plating described in that disclosure, also resulted in severe problems of scale buildup on, and etching and corrosion of the anode. The disclosure of U.S. Pat. No. 4,588,481 specifies a variety of sulfonic acids, including methanesulfonic acid (MSA), ethanesulfonic acid (ESA), methanedisulfonic acid (MDSA) and 1,2-ethanedisulfonic acid (EDSA). Generally for economic reasons, MSA has become the agent of choice in a number of commercial embodiments for chromium plating which have appeared in the marketplace, even though severe scale buildup and anodic corrosion are encountered.
As noted hereinabove, when chromium-plating processes using MSA have been installed and utilized commercially, difficulty has arisen in functional plating using lead or conventional lead-alloy anodes; investigation into the matter of anode corrosion subsequent to the issuance of U.S. Pat. No. 4,588,481 has revealed that MSA in the plating baths generally causes the excessive corrosion of those anodes after extended operation, relative to the corrosion observed in conventional plating processes.
"Conventional plating processes" or "conventional baths" are defined herein as those which are conducted with a plating bath consisting of chromic acid and sulfate ion as the essential ingredients, the sulfate ion generally being provided by sulfuric acid or sodium sulfate, although those are not limiting sources, the requirement being solely that a soluble sulfate ion be provided. It has been found that as a lead anode is used repeatedly in functional chromium electroplating with baths containing MSA, the anode disintegrates at a faster rate than in conventional baths, and it must therefore be replaced much sooner than the anode in an analogous conventional bath. In this specification, the term "lead anode" is intended to define plating-bath anodes formed of lead or lead alloys commonly containing varying percentages of tin or antimony, either alone or in combination with other metals. In the nomenclature of the art, a term such as "Pb-7%Sn" is used to designate a tin-lead composition being primarily lead, and having about 7% tin by weight as the alloying metal. In such compositions, there may further be minor amounts of other materials present. Such materials are well known to those skilled in the art, and as such form no part of the invention described herein.
In my U.S. Pat. No. 4,786,378, I introduced bismuth, arsenic or antimony ion into the bath with MSA in an attempt to reduce anode corrosion. Thereafter, in U.S. Pat. No. 4,810,337, describing the use of sulfonic acids in electroplating processes, I further disclosed one treatment of the anode-corrosion problem described here in connection with the use of MSA. In that patent, I noted that a heavy scale deposit occurs in plating processes using MSA, and applied a relatively high voltage across the electrodes prior to the plating process in order to reduce the observed scale buildup and concomitant corrosion.
Another attempted solution to the problem has been the investigation of materials which are resistant to attack by bath compositions containing MSA. For instance, in German application 3,625,187A, filed on Jul. 25, 1986, anodes made of lead containing up to about 9% by weight of antimony or about 1% by weight of palladium, with or without small amounts of tin, silver and/or selenium are reported to show "good results" when used in functional chromium electroplating processes carried out at 55.degree. C., with a cathodic current density in the range of 30 to 32 amperes per square decimeter (A/dm.sup.2) and an anodic current density of from 25 to 30 A/dm.sup.2
I have also investigated the effect of the purity of MSA on anode corrosion, on the supposition that impurities accompanying MSA might be at least a part of the problem. As noted in connection with Table II hereinbelow, this has been found not to be the case.
The foregoing publications and experimental work indicate at least in part the magnitude of the effect of anode scale and corrosion on plating, and the number and variety of approaches to its solution. However, until the evaluations leading to the present invention, workers in the art of chromium plating did not recognize that alkylpolysulfonic acids used as plating catalysts could both improve plating efficiency and decrease anode corrosion.
MSA and ESA have been generically identified as useful additives in plating baths for functional chromium-plating processes. However, as discussed hereinabove, the relevant references have indicated the problem of severe anodic corrosion when chromium is functionally electroplated for an extended period of time with lead anodes in plating baths containing MSA, the industry standard. Significantly, those references fail to suggest or disclose any particular means for an economical solution to the problem without sacrificing cost or process efficiency, or the other advantages obtained using baths containing MSA.